EncryptByKey uses a symmetric key. This key must be open. If the symmetric key is already open in the current session, you do not have to open it again in the context of the query.

The authenticator helps you deter whole-value substitution of encrypted fields. For example, consider the following table of payroll data.

Employee_ID

Standard_Title

Base_Pay

345

Copy Room Assistant

Fskj%7^edhn00

697

Chief Financial Officer

M0x8900f56543

694

Data Entry Supervisor

Cvc97824%^34f

Without breaking the encryption, a malicious user can infer significant information from the context in which the ciphertext is stored. Because a Chief Financial Officer is paid more than a Copy Room Assistant, it follows that the value encrypted as M0x8900f56543 must be greater than the value encrypted as Fskj%7^edhn00. If so, any user with ALTER permission on the table can give the Copy Room Assistant a raise by replacing the data in his Base_Pay field with a copy of the data stored in the Base_Pay field of the Chief Financial Officer. This whole-value substitution attack bypasses encryption altogether.

Whole-value substitution attacks can be thwarted by adding contextual information to the plaintext before encrypting it. This contextual information is used to verify that the plaintext data has not been moved.

If an authenticator parameter is specified when data is encrypted, the same authenticator is required to decrypt the data by using DecryptByKey. At encryption time, a hash of the authenticator is encrypted together with the plaintext. At decryption time, the same authenticator must be passed to DecryptByKey. If the two do not match, the decryption will fail. This indicates that the value has been moved since it was encrypted. We recommend using a column containing a unique and unchanging value as the authenticator. If the authenticator value changes, you might lose access to the data.

Symmetric encryption and decryption is relatively fast, and is suitable for working with large amounts of data.

Important

Using the SQL Server encryption functions together with the ANSI_PADDING OFF setting could cause data loss because of implicit conversions. For more information about ANSI_PADDING, see SET ANSI_PADDING (Transact-SQL).